With the advancement of information communication technologies, various wireless communication technologies have recently been developed. A wireless local access network (WLAN) is a technology whereby super high-speed internet access is possible in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted. Initially, WLAN used a frequency of 2.4 GHz to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared ray communication, etc. Recently, the WLAN can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplexing (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc.
The IEEE 802.11b standard supports a data rate of up to 11 Mbps (bits per second) by using a frequency band of 2.4 GHz. The IEEE 802.11a standard uses a frequency band of 5 GHz instead of the frequency band of 2.4 GHz and thus significantly reduces influence of interference. The IEEE 802.11a standard has improved the data rate to up to 54 Mbps by using the OFDM technology. The IEEE 802.11n standard provides increased network speed and reliability, extended coverage.
Basic access mechanism of an IEEE 802.11 is a carrier sense multiple access with collision avoidance (CSMA/CA) combined with binary exponential backoff. The CSMA/CA mechanism is also referred to as a distributed coordinate function (DCF) and basically employs a “listen before talk” access mechanism. A station (STA) listens a wireless medium before starting transmission. As a result of listening, if it is sensed that the wireless medium is not in use, the listening STA starts its transmission. Otherwise, if it is sensed that the wireless medium is in use, the STA does not start its transmission but enters a delay duration determined by the binary exponential backoff algorithm. The CSMA/CA channel access mechanism is not so efficient since throughput at the MAC layer provides only 50 to 60% of throughput at the physical layer.
IEEE 802.11 VHT (Very High Throughput) is one of WLAN systems which have been recently proposed to support throughput of higher than 1 Gbps. Two kinds of VHT system are independently progressed: one is IEEE 802.11ac below 6 GHz band and another is IEEE 802.11ac for 60 GHz band.
The VHT system is expected to use bandwidths broader than at least 60 MHz. A AP simultaneously transmit data at different frequencies to multiple STAs to increase the overall throughput.
A technique for efficiently operating a wideband VHT system is required.